a short part of my writing , aw >grammar remains< my biggest obstacle ;)

It is sometimes claimed that zoos should be closed because animals are suffering inhuman treatment in some commercial zoos aimed >at< entertaining people. For example, animals live in >hostile conditions<. To maximize >< profits, some zoos may cut the staff. As a result, there are >insufficient< people to keep living >environments< clean. Even worse, because of visitors’ adverse behaviors, animals >sometimes contract< diseases>.< It is difficult to receive adequate medical >care,< leading to >the< accidental death of some animals.

In addition, they may overload animals in a narrow area >to save< space. >This< is harmful for male animals>,< which have strong territoriality. Animals may >become< sick because of insanitary condition>s< and fighting. Some zoos do not have specialized medical employees for all kinds of animals>, which< probably >causes the deaths of some< animals. Besides those drawbacks, some zoos >import exotic< animals, which are >unable< to live in other countries, >to attract< visitors. Some [research shows OR researches show] that forcing animals to change their way of living may result in a decline in >their< birth rate.

I give you a B+   Considering that I can't even construct a simple sentence in the language that I'm learning.  Your grammar is very good.   It may not be technically correct sometimes, but I can easily understand what you're saying.